Digital printers are devices that are used to make hard-copy reproductions of digital images. There are many types of printers that are currently used. Ink jet printers are one common type of printer. With ink jet printers, drops of ink are deposited on a substrate after being ejected from an ink jet head. Typically, ink jet printers may print images with black ink, with cyan, magenta, and yellow (CMY) ink, or sometimes with cyan, magenta, yellow and black (CMYK) ink. For many commercially available printers, the drops of ink are deposited on the substrate on a rectangular grid at approximately 300 dots per inch (dpi). Since many ink jet devices are inherently binary in nature (a dot is either deposited, or it is not), halftoning techniques such as error diffusion and ordered dither are generally used to create the appearance of digitized continuous tone images. Although images of moderate quality can be created in this manner, they generally fall noticeably short of "photographic quality."
There are a number of ways to increase the image quality obtained from an ink jet printer. One method is to increase the number of pixels per inch that can be generated by the printer. For example, ink jet printers have been recently introduced that print images at 600 dpi or even more.
Another method to increase the image quality for an ink jet printer is to increase the number of effective density levels that can be printed for any given pixel. One way that this can be done is by varying the size of the dots that are deposited on the substrate. Some ink jet technologies allow the size of the ink drop to be adjusted continuously throughout some range. Other technologies can produce a small number of discrete dot sizes. In this case, multilevel halftoning techniques are typically used to produce the appearance of a digitized continuous tone image. For example, a printer recently introduced by Hewlett-Packard (the HP 850) produces dots of three different dot sizes on a 300 dpi grid.
Other types of printers vary the effective density level for a pixel by adjusting the amount of colorant deposited on the substrate. Thennal dye transfer printers are one such class of devices. In this case, the density for a given pixel is controlled by the amount of heat applied to a dye donor ribbon. For an ink jet printer, continuously varying the ink colorant concentration is not a practical means for adjusting the effective density level. However, the use of a small number of different inks has been shown to produce desirable benefits. Canon has disclosed several ink jet printer arrangements that use a number of inks having different colorant concentrations where, additionally, the dot sizes can be varied continuously throughout some range (for example, see U.S. Pat. Nos. 4,560,997 and 4,959,659). In these arrangements, the primary control of the density is accomplished using the dot size variation, and the multiple ink colorant concentrations are used to extend the available density range.
These prior art arrangements disclosed have no provision for creating images on a printer that is capable of producing a small number of discrete dot sizes for a set of inks varying in colorant level. This problem is significantly more complex due to the fact that the density can not be controlled by continuously varying either the dot area or the colorant concentration.